I buy and sell clothing a decent amount, while I don't do it at a high volume I don't usually stray from marketplaces because it's curated a group of buyers or sellers who have been vetted through transaction histories that they're good to deal with.

Also the consistency of a transaction, I know that there is a third party also making sure the seller tosses the tracking number on the site or remembers to ship it at all. Versus offline may be touch and go if you don't have a history with the seller or the buyer.